Two-stage pumping system with automatic demand responsive control



May 31, 1955 P. EDELMAN ETAL 2,709,339

TWO-STAGE PUMPING SYSTEM WITH AUTOMATIC DEMAND RESPONSIVE CQNTROL FiledJuly 6, 1953 s SheetsSheet 1 HYDRAULIC FLUID RESERVOIR INVENTORS:FRANKLIN RUDY 8 PHILIP EDELI'IAN,

ATTORNEYS.

, 1955 P. EDELMAN EI'AL 2,709,339

TWO-STAGE PUMPING SYSTEM WITH AUTOMATIC DEMAND RESPONSIVE CONTROL 3Sheets-Sheet 2 May 31 Filed July 6, 1953 li V May 31, 1955 P ERELMANETAL 2,709,339

TWO-STAGE PUMP s EM'WITH AUTOMATIC DEMAND RESPO VE CONTROL Filed July 6,1953 3 Sheets-Sheet 3 1 3 34 b I/IIIIIIIIII if 1 r t 33 v 5? 5 a! I. 342.4- 40 INVENTORS: FRANKLIN RUDY a PHILIP EDELNAN- A T TORNE YS.

United rates TWO-STAGE PUMPENG SYSTEM WITH AUTO- MATIC DEMAND RESPONSIVECGNTROL Application July 6, 1953, Serial No. 356,330

17 Claims. (Cl. 60-52) This invention relates to pumping systems of thetype utilizing a high pressure piston pump for delivering 'hydraulicfluid to a hydraulic motor or other device requiring a supply of highpressure operating fluid. The invention is directed particularly topumping systems operating with a two-stage pumping action derived'from asuper-charger or low pressure feed pump, feeding working fluid to theinlet of a high pressure, piston pump of a feathering type (foradjustment to varying load demand), the output of which is delivered tothe device requiring the high pressure fluid. Systems of this kind arecommonly in use in various types of apparatus where in a servo unit(such as a hydraulic servo-motor for operating a gun turret or hydraulicmechanism for actuating the landing gear or other accessories of anaircraft) 'is actuated by Working fluid supplied under pressure by thepumping mechanism, which may be located remotely from the servo unit. inmany of such installations there are a number of the servo units, allsupplied from a common pumping source. Some of the requirements of sucha system, as used for example in aircraft, include the ability todevelop high operating pressures and to withstand elevated temperatures.A primary re quirement for aircraft is compactness and minimum weight.Accordingly, a general object of the invention is to altord a compact,low weight pump, capable of developing high flow rates at high operatingpressures and of withstanding the accompanying elevated temperatures, soas to be especially suitable for airborne and missile service.Heretofore, it has been possible to obtain such performance only inheavy industrial pumps. The present invention contemplates a pumpcapable of developing pressures as high as 5000 pounds per square inch,while operating (with a suitable fluid) at temperatures up to 450Fahrenheit.

Existing pumping systems of the two-stage type with automatic featheringare characterized by the fact that the system, when operating, isconstantly subjected to the maximum output load of the supercharger. Inour system, the supercharger is designed to deliveran output in excessof the maximum requirement of the high pressure pump, yet the fulloutput load of the supercharger is not imposed on the system except atmaximum demand conditions. One of the important objects of the inventionis to provide a two-stage pumping system embodying means toautomatically unload the system at all times to whatever extent isdictated by reduction in load demands. Thus when the high pressure pumpis fully feathered, the pump will be substantially unloaded, with thesupercharger output reduced in pressure and bypassed back to reservoir.Also when the high pressure pump is operating at partial capacity, thesupercharger output pressure will be substantially reduced below itsmaximum value.

A particular object of'the invention is to provide an improvedfeathering piston pump of the multiple piston type. A further object isto provide such a pump, wherein the piston stroke is controlled inaccordance with the atet rate at which working fluid is fed to the inletof the pump.

Another object is to provide such a pump in combination with acontroller adapted to vary the rate of feed of the working fluid to thehigh pressure pump (and to thereby vary the feathering of the highpressure pump) in accordance with load demand.

Another object is to provide a feathering pump of the axial piston andwobble cam type in which variation in piston stroke is attained withoutthe necessity for varying the angle of inclination of the wobble cam toits axis. Accordingly, the invention contemplates such a pump in which aconventional adjusting mechanism .for varying the inclination of thewobble cams, is completely eliminated.

The pump of our invention, in its operation, is particularlycharacterized by an intermittent engagement of the wobble cams with thebearing ends of plural pump pistons, With means for retaining controlover the movements of the pistons when they are out of contact with thecams.

Another object of the invention is to provide a piston pump capable ofoperating at high temperatures and having improved piston expansioncompensation characteristics, such as to reduce to a minimum, loss ofvolumetric efliciency over a wide range of temperature change. Morespecifically, the invention aims to provide a pump having the ability tocompensate for .temperature-induced viscosity changes in the workingfluid, in a manner to maintain the efliciency factor at as nearly aconstant high value as possible.

A further object is to extend upwardly the practical range of operatingtemperatures of a piston "type pump.

Reference is made to the accompanying drawings, in

r which:

through the body The system in general Referring now to Fig. l of thedrawings, We have shown, as an example of a system utilizing theinvention, a hydraulic drive system in which a working fluid, maintainedin a reservoir A, is withdrawn from the reservoir by a super-charger Bwhich pumps it to the inlet of the high pressure pump C; the output ofhigh pressure pump C being delivered to a servomotor or other fluidutilizing device, indicated at D and designated LOAD; the output of highpressure pump C being also connected to the pilot unit E of a controllerwhich includes .both the pilot unit E and an unloading andvolume-control valve unit F. The pilot unit E is pressure sensitive,responding to the output pressure of pump C, which will tend to varywith changes in the load demand. For example, assuming that the loadcomprises the hydraulic actuating cylinders of an aircraft landing gear,connected to the pumping system by a suitable control valve (not shown)which is opened when the pilot desires to actuate the landing gear, thenormal condition will be one in which the valve is closed, causing thepressure to build up to its set maximum value at the outlet of highpressure pump C, and this pressure will also be applied to the pilotunit E of the control mechanism in a manner to reduce the flow ofworking fluid to the inlet of the high pressure pump C, therebyelfecting the feathering of the pump C so as to correspondingly reduceits output in accordance with the reduced demand.

The present invention makes no effort to improve upon the constructionof the supercharger B per se. Any conventional low pressure pump, suchas the gear pump which is indicated schematically in Fig. 1, may beutilized. The main requirement for pump B is that it shall be capable ofsufiiciently high volume output to exceed the maximum demand for highrate of feed of working fluid to the pressure pump C. Obviously, it mustbe capable of matching the maximum pumping rate of pump C under alloperating conditions.

To assure a pressure head of fluid delivered from supercharger B to theinlet of high pressure pump C at all times, the pump B is designed todeliver at an output rate in excess of the rate at which pump C canaccept working fluid when operating at maximum capacity. Thus under allconditions, including operation of pump C wide open, there will beavailable inlet pressure to drive the pistons of pump C to their fullyextended positions.

The axial piston pump Referring now to Fig. 2, the axial high pressurepump of our invention is particularly characterized by utilizing abalanced arrangement of plural pistons in axially opposed aligned pairs,mounted in cylinder bores 11 in a cylinder block 12 of sleeve form,which is mounted within a cylindrical bore 13 in a housing 14. Pistons10 have annular labyrinth oil grooves 15 for minimizing the escape ofworking fluid past the pistons. Such fluid control also effectslubrication of the cylinder bores 11, in-

sures a uniform pressure gradient along-the length of the pistonengagement in the cylinder and hydraulically centers the piston in thecylinder. Extending through the central passage of cylinder block 12 isa tubular drive shaft 16 which provides an inlet passageway 17. Shaft 16is journalled in roller bearings 18, 18 the outer races of which aremounted in the end portions of the cylindrical bore 13 of housing 14,and have end thrust abutting engagement with an end plate 19 closing oneend of the housing and a cap 20 at the other end of the housing.

Cap 20 is secured to the housing in any suitable manner as by means ofcap screws 21. Annular chambers 22, 22 are defined between the ends ofcylinder block 12 and end plate 19 and cap 20 respectively.

Bearings 18, 18 and other parts, hereinafter mentioned, are lubricatedby diversion of a small portion of the supercharged liquid flowing inpasasgeway 17, through lubricant ports 23, 23' in shaft 16. As will bemore fully described hereinafter, the lubricant passing through ports23, 23' will find its Way into annular chambers 22, 22'. cooling ducts24 in cylinder block 12, for cooling the latter, and will join the fluidin chamber 22 to leave the pump through an outlet 25, whence it willreturn to reservoir A through a return line 26.

When installed in a pumping system, housing 14 is fixed in a stationaryposition, utilizing mounting feet 27; and when in operation, shaft 16 isrotated by any suitable drive connection, indicated at 28.

As indicated in dotted lines in Fig. 2, supercharger pump 13 is mountedwithin end cap 20 at one end of the high pressure pump A, cap 20providing a casing for pump B. One of the gears 29 of super-charger B issplined upon shaft 16 as indicated, whereby both pumps are driven by thesame drive shaft. The controller units E, F may also be mounted withinend cap 20 as indicated in Fig. 2. A supercharger pump suction inlet 31}also isprovided in end cap 20. A suction line 36 leads to pump B fromreservoir A to suction inlet 30 (Fig. l).

Rotating with shaft 16 are a pair of wobble cams 31, 31', which may besplined on the shaft as indicated at 32,

Fluid from chamber 22 will pass through 32. Cams 31, 31' are backed upby respective end thrust bearings in the form of relatively thin washers33, 33 of compressible semi-elastic, absorbent, wear resistant materialsuch as plastic impregnated fabric or glass fiber, or soft metal fabric;each of the bearing washers 33 having a series of segmental flat bearingfaces or lands 34 separated by radial grooves 35 (see Fig. 3). Washers33, 33 in their normal, unstressed state are flat and of uniformthickness (except at grooves 35) and lands 34 are perfectly flat andco-planar. Lands 34 bear against rigid thrust washers 37, 37' which backup the bearing washers 33, 33. Washers 37, 37 are slidably keyed inhousing 14 as by means of a key 38 on the periphery of each washer,engaged in a keyway 39 in the housing. Washers 33, 33 rotate with cams31, 31.

Grooves 35 function for distributing lubricant between lands 34 and thecoacting faces of thrust Washers 37, 37'. Lubricant flowing throughpassages 23, 23' will flow into clearance spaces between the inner racesof bearings 18, 18' and end plate 19 and caps 21 respectively, thenceaxially through the bearings, thence radially inwardly through clearancespaces between the inner bearing races and thrust washers 37, 37'respectively, thence axially through the clearance spaces between theinner margins of the thrust washers and shaft 16, thence radiallyoutwardly through slots 35 in bearing washers 33, 33', thence axially inthe clearance spaces between the peripheries of the wobble cams 31, 31and bore 13, into the annular chambers 22, 22'. From chambers 22, 22,the flow returns to reservoir A, as previously explained. As the fluidtravels radially outwardly in grooves 35, it comes into contact with thebearing faces of thrust washers 37 37. As they rotate relative to thethrust Washers 37, 37, the washers 33, 33' will be compressed to formwedge shaped pockets 40 of minute depth, loaded with hydraulic fluidwhich supports the end thrust load. This is accomplished in such amanner that the load is completely supported by the lubricant filmbetween these bearing surfaces, rather than by actual physical contactof the mating bearing surfaces, in spite of the fact that they are subjected to intensely high axial pressures. As indicated in Fig. 5, thesewedge shaped pockets are complete on each bearing segment 34, and existby virtue of the semielastic, compressible, absorbent characteristics ofthe hearing material and not due to any pro-shaping of the washers. Thisresults in a novel bearing that will operate equally well regardless ofthe direction of movement and will properly adjust itself to meetvarious speed and load conditions. A suitable material for element 33 iscommercial fabric phenolic sheet, prepared with lapped flat segments 34.v

The opposite or forward faces of wobble cams 31, 31' (Fig. 2) areinclined so as to define dihedral angles relative to the back bearingfaces referred to above. These forward faces are recessed, being definedbetween inner and outer marginal flanges 41. In the annular face groovesthus formed are mounted annular cam facing washers 42, 42, which may beof a wear resistant bearing material similar to that of washers 33, 33.On the ends of pistons 10 are ball heads 43. Bearing shoes 44 (Fig. 7)are provided with sockets 45 and are coupled to pistons 10 by thereception of ballheads 43 in sockets 45. Opposite sockets 45, shoes 44are provided with annular bearing lands 46 which are flat and areadapted to bear against the cam face members 42. Defined within thelands 46, which are annular in form, are pockets 47 adapted to besupplied with oil under pressure for hydraulically balancing the shoesagainst the cam face members 42. Lands 46 may be provided with annularconcentric oil seal grooves as shown.

As shown in Fig. 6 the shoes 44 are tied together by aligning couplerrings 36, having notches or elongated holes 36a for receiving reducedmedial waist portions of the respective shoes, defined by medial annulargrooves 36b in which the coupler rings 36 are loosely fitted (suffirciently to allow tilting of ashoe 44 to a slight angle of inclination ofthe face plane of its lands 46 relative to the plane of coupler ring 36when the latter is tilted away from cam face washer 42 or 42' as shownin Fig. 2; thus allowing the lands 46 to seal flatly against the camface washer). Alternatively, the coupler ring 36 may be of asufficiently flexible material (e. g. a lightly plasticized syntheticresin thermo-plastic material hardened synthetic rubber) so that thering itself may flex in the area of the shoe that is making full contactwith the cam facing. Coupler rings 36 function to maintain theirrespective sets of shoes 44 at all times in general planar alignment,and thereby dictate the relative positions of pistons which are coupledto the shoes 44.

Mounted in radial bores in cylinder block 12, directly between the endsof cylinders 11, are ball seat cages 48 each having an outer seat 49 foran outlet valve 50 and an inner seat 51 for an inlet valve 52. Valves 50and 52 as shown, may be of the ball-check type, urged toward seatingpositions by conical coil springs 53, 54 respectively. Inlet ports 56,extending radially through the tubular wall of shaft 16, communicatewith an annular inlet passage 57 in cylinder block 12 surrounding shaft16. Annular inlet passage 57 in turn communicates with the inlet portsdefined within valve seats 51. Lateral ports 58 in cages 48 providecommunication between inlet valve seats 51 and piston cylinders 11, andalso provide con munication between the piston cylinders and outletports which are defined within outlet valve seats 49. The open outerends of cages 48 communicate with an annular outlet passage or manifold60 in housing 14. Passage 60 in turn communicates with a deliverypassageway 61, in housing 14, which is coupled with pilot valve Ethrough a duct 87, and is also (as best shown in Fig. 1) coupled to afluid pressure line 88 leading to the servo unit to be supplied by thepump. On either side of annular outlet pasasge 60, cylinder block 12 issealed to housing 14 by annular packing glands 62. Similarly, theinterior of cylinder block 12, at the respective ends thereof, is sealedto the outer face of shaft 16 by packing glands 63 disposed incounterbores in the end of cylinder block 12.

Oil under pressure is supplied to the hydraulic balancing pockets 47(best shown in Fig. 7) by capillary passages 64 extending axiallythrough pistons 10 and through ball heads 43 and communicating withsmall ports 65 in the centers of shoes 44.

Viscosity compensation One of the problems in high temperature, highpres sure piston pumpsone that has created difficulty for engineersthroughout many years of effort to overcome it, is the problem ofattempting to maintain high volumetric efliciency of the pump pistons atall times without causing the pistons to freeze in their cylinders, tobecome scored or galled, or to exercise an excessive wearing action onthe cylinder bores. Modern finishing techniques have made it possible toobserve very close tolerances in the fitting of pistons to theircylinders. This, however, does not solve the problem, which iscomplicated by two variable factors, namely (a) the change in viscosityof the working fluid in response to change in temperatures; and (b) theeffect of expansion of piston and cylinder in response to temperaturechanges.

The problem is made especially difiicult by the fact that the escape offluid through the clearance spaces between pistons and cylindersincreases directly in proportion to the increase in the length of theengaging piston surfaces and as the cube of the clearance. At extremelyhigh temperatures, the working fluids now considered suitable for use inhydraulic drive systems, (usually a hydrocarbon oil) approach theviscosity of water, although most of them are fairly viscous at lowtemperatures.

The present invention deals with this problem by utilizing, as thematerial for the pistons, a metal alloy which expands at a faster ratethan the metal of the cylinder block, whereby the clearance betweenpistons and cylinders will decrease as temperature increases.

More specifically, the invention provides for select-ing a ratio betweenthe 'coeflicient of expansion of the piston and that of the cylinder,such that the decrease in clearance will be so proportioned totemperature increase as to compensate for the thinning of the workingfluid under such temperature increase. The optimum result aimed at bythe invention is to proportion the value of clearance decrease to thevalue of viscosity decrease so as to approximate a constant ratiobetween these variables, and at the same time to cause the clearancebetween pistons and cylinders to decrease as temperature increases, butwithout binding.

More specifically, the invention provides for selecting a ratio betweenthe coetficient of expansion of the pistons and that of the cylinders,such that the decrease in clearance will be so proportioned totemperature increase as to compensate for the thinning of the workingluid under such temperature increase. The optimum result aimed at by theinvention is to so proportion the curve of clearance decrease to thecurve of viscosity decrease as to approximate a constant ratio betweenthese variables, and thereby to compensate for the temperature-inducedchange in the fluid escape factor referred to above, in a manner tomaintain the pumps volumetric efiiciency at a constant high value.

As an example of a reduction to practice of this concept which we havefound to be highly "successful in approaching this optimum goal, thecylinder block may be constructed of steel with cylinder walls hardenedby nitriding, and the pistons may be constructed from heat treated,hardened, beryllium-copper. The respective coefficients of expansion ofthese materials are such as to achieve the proportionate decrease inpiston clearance in step with temperature increase, as outlined above.As another example, the pistons could be constructed of hardened steeland the cylinder block of another alloy having a lower coefficient ofexpansion than hardened steel, such as Invar metal.

The primary advantage obtained by this improvement is in providing for asubstantial increase in volumetric efficiency at elevated operatingtemperatures. In accordance with the results indicated by tests that wehave made on the preferred example given above, the invention makes itpossible to obtain a 50% increase in operating temperatures over themaximum temperatures at which it has been possible in the past tooperate piston pumps without reducing efiiciency below minimumrequirements.

In addition to the use of metals having the novel expansioncharacteristics indicated above, the invention is further characterizedby fitting the pistons to their cylinders with minimum clearance at themaximum operating temperature, after having determined in advance theminimum clearance which can be tolerated without encountering the dangerof freezing or galling of the pistons in the cylinders. More clearanceis satisfactory at lower temperature because the working fluid is thenmore viscons. Accordingly, at minimum temperatures (normal atmospherictemperatures) the pistons have maximum clearance in the cylinders,corresponding to maximum viscosity of the working fluid. As temperatureincreases, piston clearance will decrease in step with decrease inviscosity of the working fluid.

The sealing effectiveness of the labyrinth sealing grooves 15 willincrease as the clearance decreases. We find that by determining theminimum clearance at maximum temperature, thereby avoiding aninterference fit at any temperature, there is a complete avoidance,throughout the range of operation of the pump, of any galling of'thepistons or cylinders, and thes'urfaces of these parts simply becomepolished and burnished to a high degree, which extends the higheificiency of opera tion of the pump indefinitely. H

A particular advantage arising from the novel use of beryllium-copper asthe material for the pistons, is the high heat conductivity of thatmaterial, which avoids any localized hot spots on the piston surfaces orcylinder walls and thereby promotes uniformity of clearance throughoutthe engaging areas of the pistons and cylinders.

Volume control and bypass valve F As best shown in Fig. l, superchargedfluid is delivered from the outlet of supercharger B through a fluidline 66 to a passage 66 in the casing (a portion of end cap 20) of valveunit F. Valve unit F provides two separate paths, both branching frompassages 66', for bypassing excess working fluid back to reservoir A.Normal by-passing, to dispose of supercharged fluid rejected by thepressure pump C when operating at less than full capacity, is through aspool valve bore 67 in casing 20', past a piston 68 of a double pistonspool valve which is slidable in bore 67, into an annular port 69communicating with bore 67, thence into a return passage 70' leadingdirectly back to the reservoir through a return line 70. Such bypassingoccurs at pressures below maximum supercharger outlet pressure.

When the supercharger outlet pressure tends to exceed its maximum value(when pump C is operating at full capacity) excess fluid will berelieved through a pressure relief port 71 past a pressure relief valve72, into return passage 70. Relief valve 72 may be of the poppet type asshown. A loading spring 73, adjustably backed up by an abutment screw74, determines the pressure at which valve 72 will open port 71. Therelief poppet 72 will establish the normal supercharger operatingpressure. It is intended to operate only when the main piston pump C isin a maximum displacement, or nonfeathered position. Since thesupercharger pump is designed to have a capacity slightly in excess ofthe piston pump C, only this excess fluid will be bypassed throughrelief valve 72.

A second piston 75 is connected to piston 68 by a stem '76 to form thecomplete spool valve element. The inner edge of piston 75 controls twosemi-circular ports 77, 78

respectively in the casing 20. Port 77 provides for passage of fluidfrom the passageway 66 into a central chamber 79 defined between thepistons 68 and 75, within the spool valve bore 67. The operation ofpiston 75 in throttling port 78 controls the rate of flow of fluid fromchamber 79 through a connecting line 80 to the inlet passage 17 of thehigh pressure pump C. Ports 77, 78 are slightly offset; port 77remaining open when piston 75 has closed off port 78.

Piston 68 controls the escape of fluid from chamber 79 through annularport 69, into return passage 70'. As the two pistons move leftwardly asviewed in Fig l, the aperture of port 78 will be decreased while that ofport 69 is increased, thus bypassing an increasing amount of fluidthrough the reservoir and decreasing the flow through line 80 to pump C.

The movement of spool valve 68, 75 is governed by the opposed forces offluid pressure acting against the outer end of piston 75 and a spring 81acting against piston 68. Spring 81 is adjustably backed up by anabutment screw 82, the axial position of which may be varied so as tovary the positions occupied by spool valve 68, 75 in response to changesin the discharge pressure of pump C. By advancing screw 82, thedischarge pressure of pump C required to shut cit flow through line 80will be increased. Screw 82 may be pre-set and locked in place (as bystaking).

The valve unit F actually comprises two separate valvesgi. e., thevolume control valve 67-.-69, 7582, and the bypass valve 7174, thelatter being located between the supercharger B and the volume controlvalve. Although, for compactness, these valves are illustrated as beingcombined in a single unit, they could be physically separated, it beingonly necessary to extend ports 70' and 66' into passages of adequatelength to connect the two valves. Similarly, the valve units E and F,though shown in Fig. 2 as being built into a common casing, could bephysically separated as suggested in Fig. 1.

Obviously, in order to function as intended, the bypass valve 7174 mustopen at a pressure lower than the maximum rated output pressure ofsupercharger B.

T he latter pressure need only be high enough to maintain flow toconduit 80 when port 78 has been throttled down to such restriction aswill result in minimum flow to pump C, and to effect opening of inletvalves 50. This pressure may be quite low as compared to the pressuresdeveloped by pump C, and thus the unloading is effective to minimizeworking losses.

Volume control pilot E The volume control pilot E embodies a casing 85having a bore 86 which communicates, through a fluid line 87, with thedelivery line 88 through which working fluid is conveyed from the outletof pressure pump C to the servomotor or other device constituting theload D. The piston 89, slidable in bore 86, is subjected at its outerend to the discharge pressure of pump C through line 87, its inner endtransmitting this pressure to a spring 90 through a spring retainer cup91 having a stem 92 against which the inner end of piston 89 engages.Retainer cup 91 is slidably mounted in a differential pressure chamber93 in casing 85, into which pressure fluid from line 87 is introducedthrough a passage 94 in casing 85, to reach the rear side of cup 91,thence through ports 95 in cup 91 as indicated by the arrows. A transferline 96, branching from passage 94, leads to a fluid chamber 97 in therear end of the volume control valve F, and acts upon the volume controlpiston 75 of valve F in accordance with the pressure in passages 94 and96.

The pressure in passage 94 is a function of the pressure in line 87 andis also a function of the rate at which fluid is bled out of chamber 93of pilot unit E past a needle type bleed valve 101, the lattercontrolling the bieeding of fluid from chamber 93 through a port 102 toa radial passage 103 in a plug 104 (constituting a casing for the needlevalve 101). From an annular peripheral groove 105 in plug 104, withwhich passage 103 communicates, the fluid which has bled past needlevalve 101 escapes through a passage 106 in pilot casing 85 and thencepasses through a return line 107 which may join the return line 108leading from the servomotor D to the reservoir, as shown in Fig. 2, orthe return line 26, as shown in Fig. l, or may join the suction line 30to pump B.

The apertures between needle valve 101 and port 102 may be varied byoperating an adjusting screw 110, threaded into sleeve 104. the needlevalve 101 constituting an end portion of screw 110. Further adjustmentof the pilot unit E is provided for by an adjusting knob 111 having athreaded connection with casing 85 and formed as an integral head on theouter end of sleeve 104. By rotating knob 111, sleeve 104 is axiallyshifted in chamber 93, and since one end of spring 90 abuts against theinner end of sleeve 104, the compression loading of the spring isthereby varied. Correspondingly, the spring load which is applied to theinner end of piston 89 by stem 92 will be varied, so that the hydraulicpressure required to operate the pilot piston 89 (to adjust the pumpfeathering pressure) may be externally adjusted. it will now be apparentthat piston 89 operates as a slide valve to vary the effective area ofport 94', thereby varying the relation between the pressure in line 87and the pressure in line 96. The said lines may be wholly or partly inthe form of drilled passageways in the pump body, as indicated in Fig.2.

Referring to the pilot E as shown in Fig. l, the area of port 94 willvary in accordance with fluctuations in the discharge pressure of pumpC. The pressure in differential pressure chamber 93, passage 94,transfer line 96 and chamber 97 of volume control valve F is governed bythe ratio between the rate of passage of fluid into passage 94 throughport 94 and the escape of fluid past needle valve 101 into return line107, and is therefore a function of the discharge pressure of pump Cwhich governs the area of port 94. This function can be adjusted, bothas to pressure and rate of feathering, by actuating adjusting knob Friand adjusting screw 110 respectively.

The pressure pump C of our invention is particularly characterized byits ability to achieve feathering or piston stroke adjustment inresponse to changes in its own discharge pressure, without utilizingconventional tiltable mountings of the wobble cams and pressureresponsive means for adjusting the inclination of the cams. The cams ofour pump are fixed to shaft 16 with fixed angles of inclination of theirhearing faces 42, and, except at maximum piston displacement, the shoes44 will contact the earn faces only during a portion of their paths ofrotary movement. The contacts will be made of course in the high regionsof the cam faces, and in the low areas thereof the shoes will draw awayfrom the cam faces.

The factor which governs the piston stroke is the rate of delivery offluid into cylinders 11 through inlet ports 58. This delivery rate, inturn, is controlled by the operation of volume control valve F inresponse to changes in the discharge pressure of pump C, varying the proportion of the output of super-charger B which is delivered into line 89and the proportion that is bypassed through return line 79.

Planar substantially circular paths of travcl of shoes 54 are dictatedby coupler rings 36, best shown in Fig. 6. The retracted position of apiston it) is always the same, being determined by engagement of a shoe44 against the high point of the cam face, as best shown in 2. As theshoe 44 passes the high point, fluid pressure in inlet port 58 will actagainst piston 1) to push it outwardly, so that the shoe 44 tends tofollow the cam face down the incline toward its low point (point ofmaximum separation of shoes 44 from the cam face).

Similar action takes place sequentially in the other cylinders. However,the outward movement of pistons 10, directly governed by the rate ofinlet flow, will not be fast enough to maintain the shoes in contactwith the cams except during maximum volume operating conditions.Consequently, the shoes, after having been pushed inwardly to the fullyretracted positions by engagement with the cams, will separate from thecam faces and will follow paths which are inclined at a lower angle ofinclination than that of the cam faces, the divergence between theseangles increasing with a decrease in the rate of inlet flow.

Inlet and outlet valves 52 and 50, being spring controlled check valves,will move in accordance with the conditions of fluid pressure affectingthem. As a piston is pushed inwardly by cam operation, the pressure offluid at the inner end of the piston will quickly build up to a valuesufficiently near that of pressure in inlet passage 17, so as to causeinlet valve 52 to be closed by its operating spring. This build up inpressure will increase the pressure tending to unseat outlet valve 56until the spring 54 yields and allows valve 50 to open, whereupon thefluid pushed ahead of pistons 14 will be expelled through the outletports into the outlet manifold 69, thence to the trunk outlet 61 fromwhich it is delivered to the delivery line 88.

As the shoes 44 pass the high points of the cams, the cam pressureagainst the pistons will be relieved and fluid pressure at the ends ofthe pistons will quickly drop to the point where outlet valves 50 willclose and inlet valves 52 will open in response to the pressure in inletpassage 17. This stage of operation is shown in Fig. 2. Because of thepressure drop across inlet valves 52, the pressure of fluid passing theinlet valves and entering inlet ports 58 will be insuflicient to unseatoutlet valves 50. It will however, be sufficient to return pistons ropening passage 78.

10 outwardly at a rate corresponding to the rate of inlet flow. Thus thepiston stroke is fully controlled by the rate of inflow, and acorresponding rate of discharge from the outlet 61 will be observed. 1

The pump will operate with any suitable hydraulic fluid. No means forextending the pistons agains the wobble cams, other than thesupercharger pressurized fluid, is required in the pump of ourinvention. Since the suction or return stroke is not mechanicallyactuated, but rather is affected by the low pressure fluid from thesupercharger pushing the pistons against the wobble cams and filling thecylinder with fluid, it is impossible for the piston pump to cavitate.Since the supercharger pump is working only under relatively light loadsit is of such a design as to be capable of operating under very lownegative pressure heads (i. e. high suction) on the suction port; andcould resemble a conventional vacuum pump. An cavitation that may takeplace would then occur (harmlessly) at the supercharger pump. Since thesupercharger pump does not require as exact machining tolerances etc.,the phenomenon of cavitation may more readily be tolerated at this pointin the circuit. Because of the aforementioned feature, emcientabnormally high altitude operation, required for modern aircraft usage,is attained by the invention.

Since the shoes 44 are out of contact with the wobble cams during amajority of their operating time, wear on the shoes is reduced to aminimum.

The bleeding of pressure fluid through piston passages 64, which are ofcapillary dimensions, serves to produce line high velocity jets throughports 65, which jets will impinge upon cam facings 42 as the shoes 44approach the same, thereby cushioning the impact of the shoes againstthe facing. As the peripheral lands of a shoe approach contact with thecam facing sufliciently closely to restrict the escape out of chamber 47of the fluid coming from this jet, the cushioning action is built up inchamber 47 so that the actual contact of the shoe with the facing 42 iscompletely devoid of slap or jar, and there can be no rebound orchattering action under these conditions. Accordingly, the intermittentcam action is smooth, gradual, and lacking in vibration or chatter. Asthe shoe makes contact, it will adjust its position to one of flat faceto face contact between its lands and the cam facing, tilting slightly,if necessary, in order to do so.

The operation of volume control valve F may be made extremely sensitivebecause of the pressure reduction which occurs across port 94' of pilotunit E. The metering action of piston 89 in pilot unit E, in cooperationwith the bleed past valve 101, reduces the extremely high pressure inline 87 to a low value of operating pressure in line 96 which may beadjusted to exactly the right value for optimum performance of valve F,while main? taining a fixed ratio between the high pressure fluctuationsin line 87 andthe induced low pressure fluctuations in line 96. v Animportant aspect of the invention is the provision for unloading thesystem. Although the supercharger B provides a constant output in excessof maximum demand of pump C, its maximum output pressure will beattained onlywhen pump C is operating at full capacity. Pressure inlines 87, 96 will then be at a minimum value, and spool valve 68, willbe moved to an extreme rightward position, completely shutting offbypass 7d and fully At such. times, relief poppet 72 comes intooperation to effect high-pressure relief bypassing of excess fluid. Whenload demand drops off (e. g., as by restricting passage 88) pressure inlines 87 and 96 will increase, spool valve 68, 75 will be drivenleftward, opening bypass 70 and dropping the pressure in lines 66 and 88so as to reduce the loading between the outlet of the supercharger andthe inlet of pump C. In contrast, conventional two-stage pumping systemsmaintain maximum supercharger outlet pressure imposed on the pressurepump inlet at all times. Our improvement ac,-

cordingly reduces the power consumed at drive element 28 of the pumpbecause the pump combination is not required to dissipate a large amountof power in the form of heat while its high pressure pistons operate atminimized strokes. Such economical operation is possible during a majorportion of the cruise of a modern airplane but the pump is neverthelessalways in readiness to serve whenever and as much hydraulic power as isnecessary, up to its maximum capacity or any fraction of this amountthat may be required.

In referring to the paths of travel of shoes 44 as being substantiallycircular, it is to be understood that these paths are almost circular,or slightly elliptical.

The term constant output as used herein to refer to the supercharger, isto be understood to mean constant output for any given speed ofoperation of the supercharger. Stated somewhat differently the termindicates that the supercharger is a constant displacement pump.

We claim:

1. In a two-stage hydraulic pumping system, in combination: a constantoutput, low pressure supercharger; a variable output, automaticallyfeathering high pressure pump; a combined volume control and bypassvalve unit having an inlet port receiving fluid from the outlet of saidsupercharger, and having a delivery outlet and a bypass output of saidhigh pressure pump to a load and for returning spent fluid from the loadto the inlet of said supercharger; said volume control and bypass valveunit including a pressure responsive valve for metering the fiow fromits said inlet to said connecting line and bypassing the excess to saidbypass outlet, and a pressure responsive bypass valve in said bypass,said bypass valve being adapted to open in response to superchargeroutlet pressure at a selected maximum value thereof, and to limit atsaid maximum value, the outlet load of the super charger by bypassingback toward the supercharger inlet, that portion of the superchargeroutput which is in excess of the amount accepted by the high pressurepump; and a pilot valve responsive to fluctuations in the outputpressure of said high pressure pump, for supplying to said volumecontrol valve, pressure fluid derived from said high pressure pumpoutput, for actuating said volume control valve in a manner to throttlethe flow from the supercharger to the high pressure pump so as tofurnish to the latter only that quantity of working fluid which isrequired to meet the load demand.

2. In a two-stage hydraulic pumping system, in combination: a constantoutput, low pressure supercharger; a variable output, automaticallyfeathering high pressure pump; a volume control unit having an inletport receiving fluid from the outlet or" said supercharger and having adelivery outlet; a connecting fluid line extending from said deliveryoutlet to the inlet of said high pressure pump; means defining fluidpassages for delivering the output of said high pressure pump to a loadand for returning spent fluid from the load to the inlet of saidsupercharger; said volume control unit including a pressure responsivevalve for metering the flow from its said inlet to said connecting lineand bypassing the excess to said bypass outlet, said valve controllinginlet pressure of the high pressure pump when the latter is operating ina feathered condition; means defining a bypass from the outlet to theinlet of said supercharger, bypassing said volume control unit, highpressure pump and load; a pressure responsive bypass valve in saidbypass, said bypass valve being in response to supercharger outletpressure at a selected maximum value thereof, and to limit at saidmaximum value, the outlet load of the supercharger by bypassing backtoward the supercharger inlet, that portion of the supercharger outputwhich is in excess of the amount accepted by said high pressure pump;and a pilot valve responsive to fluctuations in the output pressure ofsaid high pressure pump, for applying to said volume control unit,pressure fluid derived from said high pressure pump output, foractuating said volume control unit in a manner to throttle the flow fromthe supercharger to the high pressure pump so as to furnish to thelatter only that quantity of working fluid which is required to meet theload demand.

3. In a two-stage hydraulic pumping system, in combination: a constantoutput, low pressure supercharger; a variable output, automaticallyfeathering high pressure pump; a volume control unit having an inletport, a delivery outlet and a bypass outlet; a conduit for conductingfluid from the outlet of said supercharger to said inlet port; aconnecting fluid line extending from said delivery outlet to the inletof said high pressure pump; means defining fluid passages for deliveringthe output of said high pressure pump to a load and for returning spentfluid from the load to the inlet of said supercharger; said volumecontrol unit including a pressure responsive valve for metering the flowfrom its said inlet to said connecting line while bypassing the excessto said bypass outlet, said valve controlling inlet pressure of the highpressure pump when the latter is operating in a feathered condition;means defining a bypass flow path from said bypass outlet to saidsupercharger inlet; a bypass leading from said supercharger outletconduit to said bypass path, bypassing said volume control unit; apressure responsive bypass valve in said bypass, adapted to open inresponse to supercharger outlet pressure at a selected maximum valuethereof, and to limit at said maximum value, the outlet load of thesupercharger by bypassing back toward the supercharger inlet, thatportion of the supercharger output which is in excess of the amountaccepted by said high pressure pump; and a pilot valve responsive tofluctuations in the output pressure of said high pressure pump, forsupplying to said volume control unit, pressure fluid derived from saidhigh pressure pump output, for actuating said volume control unit in amanner to throttle the flow from the supercharger to the high pressurepump so as to furnish to the latter only that quantity of working fluidwhich is required to meet the load demand.

4. in a two-stage hydraulic pumping system, in cornbination: a constantouput, low pressure supercharger; a variable output, automaticallyfeathering high pressure pump; a volume control unit comprising a casinghaving a here, an end member closing one end of said here and an axiallyadjustable abutment member closing the other end of said bore, saidcasing further having an inlet and a delivery outlet both spaced fromsaid end member, and a bypass outlet spaced from said inlet in thedirection away from said end member, said volume control unit furtherincluding a valve comprising acontrol piston, a bypass piston and a stemconnecting said pistons in axially spaced relation, said valve beingslidable in said bore with said control piston disposed between said endmember and said inlet and delivery outlets and with said bypass pistondisposed between said bypass outlet and said adjustable abutment member,and a spring interposed between said bypass piston and said adjustableabutment member and loading said valve, one end of said control pistoncooperating with said end member to define in said bore a controlchamber, in which fluid pressure may act against said control piston tobalance the load of said spring and to shift the valve in response tofluid pressure changes in said control chamber, the. other end of saidcontrol piston being disposed for throttling said delivery outlet inresponse to pressure increase in said control chamber, said bypasspiston being disposed for throttling said bypass outlet as said deliveryoutlet is opened, the space between said pistons providing for fluidflow from said inlet to said delivery outlet and to said bypass outlet;a conduit for conducting fluid from. the outlet of said supercharger tosaid inlet port; a connecting fluid line extending from said deliveryoutlet to the inlet of said high pressure pump; means defining fluidpassages for delivering the output of said high pressure pump to a loadand for returning spent fluid from the load to the inlet of saidsupercharger; said volume control unit functioning to meter the flowfrom its said inlet to said connecting line While bypassing the excessto said bypass ouhet; means defining a bypass flow path from said bypassoutlet to said supercharger inlet; a bypass leading from saidsupercharger outlet conduit to said bypass path, bypassing said volumecontrol unit; a pressure responsive bypass valve in said bypass andadapted to open so as to unload the supercharger at a selected maximumoutput pressure thereof by bypassing back toward the supercharger inlet,that portion of the supercharger output which is in excess of the amountaccepted by said high pressure pump; and a pilot valve responsive tofluctuations in the output pressure of said high pressure pump, forsupplying to said volume control unit, pressure fluid derived from saidhigh pressure pump output, for actuating said volume control unit in amanner to throttle the flow from the supercharger to the high pressurepump so as to furnish to the latter only that quantity of working fluidwhich is required to meet the load demand.

5. In a two-stage hydraulic pumping system: an axial piston typeautomatically feathering, variable output, high pressure fluid pumpcomprising a drive shaft; a casing having therein a plurality ofcylinders arranged in axially opposed groups comprising pairs of axiallyopposed cylinders, circumferentially spaced annular array, coaxial withand surrounding said shaft; pistons in said cylinders arranged in groupscorresponding to the cylinder groups; a pair of wobble cams surroundingand secured to said shaft and disposed at opposite ends of saidcylinders, said cams having annular piston-operating faces disposed inplanes that are inclined in opposite directions with reference to theaxis of said shaft, with fixed angles of inclination, and with theirrespective high points disposed in axially opposed alignment; bearingshoes attached to the outer ends of the respective pistons andengageable against said cam faces with only intermittent contact withthe high spots thereof when the pump is operating at less than fullcapacity; said casing having an inlet and a delivery outlet; a pluralityof valve units each interposed between adjacent ends of the cylinders ofa respective pair and embodying an inlet valve disposed for controllingflow from said inlet to respective cylinders and an outlet valvedisposed for controlling flow from respective cylinders to said outlet,said valves being spring biased toward their closed positions; a lowpressure supercharger for delivering to said inlet a variable flow offluid which is operable to successively move apart the pistons ofsucceeding pairsof cylinders, with the stroke of the pistons varyingdirectly in response to the rate of delivery of the fluid to said inlet;means for effecting relative rotation between said cylinders and cams,whereby to cause each pair or" pistons to be successively returned intoits cylinders to effect a high pressure pumping action wherein the fluidis forced through the respective outlet valve to said casing outlet; avolume control unit having an inlet port receiving fluid from the outletof said supercharger and having a delivery outlet; a connecting fluidline extending from said delivery outlet to the inlet of said highpressure pump; means defining fluid passages for delivering the outputof said high pressure pump to a load and for returning spent fluid fromthe load to the inlet of said supercharger; said volume control unitincluding a pressure responsive valve for metering the flow from itssaid inlet to said connecting line; means defining a bypass from theoutlet to the inlet of said supercharger, bypassing said volume controlunit, high pressure pump and load; a pressure responsive bypass valve insaid bypass and adapted to open so as to unload the supercharger at aselected maximum output pressure thereof by bypassing back toward thesupercharger inlet, that portion of the supercharger output which is inexcess of the total amount that is accepted for handling by said highpressure pump; and a pilot valve responsive to fluctuations in theoutput pressure of said high pressure pump, for supplying to said volumecontrol unit, pressure fluid derived from said high pressure pumpoutput, for actuating said volume control unit in a manner to throttlethe flow from the supercharger to the high pressure pump so as tofurnish to the latter only that quantity of working fluid which isrequired to meet the load demand.

6. An axial piston type automatically feathering variable output fluidpump comprising: a drive shaft; a casing having therein a plurality ofcylinders arranged in axially opposed groups comprising pairs of axiallyopposed cylinders, in circumferentially spaced annular array, coaxialwith and surrounding said shaft; pistons in said cylinders arranged ingroups corresponding to the cylinder groups; a pair of wobble camssurrounding and secured to said shaft and disposed at opposite ends ofsaid cylinders, said cams having annular piston-operating faces disposedin planes that are inclined in opposite directions with reference to theaxis of said shaft, with fixed angles of inclination, and with theirrespective high points disposed in axially opposed alignment; bearingshoes attached to the outer ends of the respective pistons andengageable against said cam faces with only intermittent contact withthe high spots thereof when the pump is operating at less than fullcapacity; a pair of coupling means each coupling together the shoes of arespective group, for constraining the group of shoes to remain in acommon plane while tilting to varying angles of inclination; said casinghaving an inlet and a delivery outlet; a plurality of valve units eachinterposed between adjacent ends of the cylinders of a respective pairand embodying an inlet valve disposed for controlling flow from saidinlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet, said valvesbeing spring biased toward their closed positions; means for deliveringto said inlet a variable flow of fluid which is operable to successivelymove apart the pistons of succeeding pairs of cylinders, with the strokeof the pistons varying directly in response to the rate of delivery ofthe fluid to said inlet; and means for effecting relative rotationbetween said cylinders and cams, whereby to cause each pair of pistonsto be successively returned into its cylinders to effect a high pressurepumping action wherein the fluid is forced through the respective outletvalve to said casing outlet.

7. An axial piston type automatically feathering variable output fluidpump comprising a drive shaft; a casing having therein a plurality ofcylinders arranged in axially opposed groups comprising pairs of axiallyopposed cylinders, in circumferentially spaced annular array, coaxialWith and surrounding said shaft; pistons in said cylinders arranged ingroups corresponding to the cylinder groups; a pair of wobble camssurrounding and secured to said shaft and disposed at opposite ends ofsaid cylinders, said cams having annular piston-operating faces disposedin planes that are inclined in opposite directions with reference to theaxis of said shaft, with fixed angles of inclination, and with theirrespective high points disposed in axially opposed alignment; therespective pistons having at their outer ends, means engageable againstsaid cam faces with only intermittent contact with the high spotsthereof when the pump is operating at less than full capacity; a pair ofcoupling means each coupling together the pistons of a respective group,for constraining the ends of the pistons of the group to remain in acommon plane while tilting to varying angles; said casing having aninlet and a delivery outlet; a plurality of valve units each interposedbetween adjacent ends of the cylinders of a respective pair andembodying an inlet valve disposed for controlling flow from said inletto respective cylinders and an outlet valve disposed for controllingflow from respective cylinders to said outlet, said valves being springbiased toward their closed positions; means for delivering to said inleta variable flow of fluid which is operable to successively move apartthe pistons of succeeding pairs of cylinders, with the stroke of thepistons varying directly in response to the rate of delivery of thefluid to said inlet; and means for effecting relative rotation betweensaid cylinders and cams, whereby to cause each pair of pistons to besuccessively returned into its cylinders to effect a high pressurepumping action wherein the fluid is forced through the respective outletvalve to said casing outlet.

8. An axial piston type automatically feathering variable output fluidpump comprising a drive shaft; a casing having therein a plurality ofcylinders arranged in axially opposed groups comprising pairs of axiallyopposed cylinders, in circumferentially spaced annular array, coaxialwith and surrounding said shaft; pistons in said cylinders arranged ingroups corresponding to the cylinder groups; a pair of wobble camsencircling and secured to said shaft and disposed at opposite ends ofsaid cylinders, said cams having annular piston-operating faces disposedin planes that are inclined in opposite directions with reference to theaxis of said shaft, with fixed angles of inclination, and with theirrespective high points disposed in axially opposed alignment; therespective pistons having at their outer ends, means engageable againstsaid cam faces with only intermittent contact with the high spotsthereof when the pump is operating at less than full capacity; saidcasing having an inlet and a delivery outlet; a plurality of valve unitseach interposed between adjacent ends of the cylinders of a respectivepair and embodying in inlet valve disposed for controlling flow fromsaid inlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet, said valvesbeing spring biased toward their closed positions; means for deliveringto said inlet a variable flow of fluid which is operable to successivelymove apart the pistons of succeeding pairs of cylinders, with the strokeof the pistons varying directly in response to the rate of delivery ofthe fluid to said inlet; and means for effecting relative rotationbetween said cylinders and cams, whereby to cause each pair of pistonsto be successively returned into its cylinders to effect a high pressurepumping action wherein the fluid is forced through the respective outletvalve to said casing outlet.

9. A pump as defined in claim 8, including a cylinder block of sleeveform having a plurality of axial bores constituting said cylinders, andhaving a plurality of radial recesses disposed between adjacent ends ofpaired cylinders; said valve units each including a cage havingrespective inlet and outlet ports and valve seats encircling the same,said ports aligned on a common radial axis substantially coaxial withthe respective recess, and said cages each having a pair ofdiametrically opposed lateral ports communicating with respectivecylinder bores and with the space within the cage between the respectivevalve seats, said recesses being open at the periphery of said cylinderblock and said cylinder block being removable endwise from said casingto provide access to said valve units.

10. A pump as defined in claim 9, wherein said casing has, intermediateits ends, an internal annular groove cooperating with the outer wall ofsaid cylinder block to define an annular discharge port whichcommunicates with all of said valve units to provide a common outlettherefor, and wherein said cylinder block has an internal annular recesscooperating with said shaft to define r an annular inlet portcommunicating with the inlet ports of said valve units to provide acommon inlet therefor.

11. A pump as defined in claim 10, wherein said shaft is tubular,providing therein an inlet passage, is connected to said casing inlet atone end, and has a radial 16 port for fluid flow from said inlet passageto said annular inlet port.

12. An axial piston type automatically feathering variable output fluidpump comprising a drive shaft; a casing having therein a plurality ofcylinders arranged in axially opposed groups comprising pairs of axiallyopposed cylinders, in circumferentially spaced annular array, coaxialwith and surrounding said shaft; pistons in said cylinders arranged ingroups corresponding to the cylinder groups; a pair of wobble camsencircling and secured to said shaft and disposed at opposite ends ofsaid cylinders, said cams having annular piston-operating faces disposedin planes that are inclined in opposite directions with reference to theaxis of said shaft, with fixed angles of inclination, and with theirrespective high points disposed in axially opposed alignment; therespective pistons having at their outer ends, means engageable againstsaid cam faces with only intermittent contact with the high spotsthereof when the pump is operating at less than full capacity; saidcasing having an inlet and a delivery outlet; a plurality of valve unitseach interposed between adjacent ends of the cylinders of a respectivepair and embodying an inlet valve disposed for controlling flow fromsaid inlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet, said valvesbeing spring biased toward their closed positions; means for deliveringto said inlet a variable flow of fluid which is operable to successivelymove apart the pistons of succeeding pairs of cylinders, with the strokeof the pistons varying directly in response to the rate of delivery ofthe fluid to said inlet; said shaft being journalled in said casing forrotating said cams, whereby to cause each pair of pistons to besuccessively returned into its cylinders to effect a high pressurepumping action wherein the fluid is forced through the respective outletvalve to said casing outlet,

13. A pump as defined in claim l2, including antifriction bearingsinterposed between said cams and the respective ends of the casing, saidbearings having outer races mounted in the casing, inner races in whichthe shaft is mounted, and rolling bearing elements interposed betweensaid races; said shaft being tubular to provide a fluid inlet passage,having one end communicating with said casing inlet, and having at itsrespective ends, fluid ducts extending from said inlet passage intocommunication with said antifriction bearings and said cams, forlubricating the same.

14. In a two-stage hydraulic pumping system: an axial piston typeautomatically feathering, variable output, high pressure fluid pumpcomprising a casing, a drive shaft journalled therein for rotation; saidcasing having therein a plurality of cylinders arranged in axiallyopposed groups comprising pairs of axially opposed cylinders incircumferentially spaced annular array, coaxial with and surroundingsaid shaft; pistons in said cylinders arranged in groups correspondingto the cylinder groups; a pair of wobble cams encircling and secured tosaid shaft and disposed at opposite ends of said cylinders, said camshaving annular piston-operating faces disposed in planes that areinclined in opposite directions with reference to the axis of saidshaft, with fixed angles of inclination, and with their respective highpoints disposed in axially opposed alignment; the respective pistonshaving at their outer ends means engageable against said cam faces foractuation of the pistons as the cams rotate; said casing having an inletand a delivery outlet for hydraulic fluid; a plurality of valve unitseach interposed between adjacent ends of the cylinders of a respectivepair and embodying an inlet valve disposed for controlling flow fromsaid inlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet, said valvesbeing spring biased toward their closed positions; means for deliveringto said inlet a variable flow of fluid which is operable to successivelymove apart the pistons of succeeding pair of cylinders, with the strokeof the pistons varying directly in response to the rate of delivery ofthe fluid to said inlet; means for effecting relative rotation betweensaid cylinders and cams, whereby to cause each pair of pistons to besuccessively returned into its cylinders to effect a high pressurepumping action wherein the fluid is forced through the respective outletvalve to said casing outlet; a pair of bearing washers of syntheticresin plastic sheet material having a resiliency and wear resistanceequivalent to that of the phenolic aldehyde resins, said washers havingradial hearing faces provided with face grooves extending radially fromtheir inner to their outer margins and separating said bearing facesinto radial bearing lands, and said bearing washers having theiropposite radial faces mounted against the respective cams; metal thrustwashers fixed in the respective ends of said casing and having radialbearing faces in bearing engagement with said bearing lands; and meansproviding a path for flow of hydraulic fluid to the inner extremities ofsaid grooves, said lands, as they rotate against said thrust washers,being slightly compressed by the lubricant entering between theirleading edges and the thrust washer bearing faces, whereby there isformed a plurality of wedge-shaped spaces of minute depth, loaded withhydraulic fluid which supports the end thrust loads developed betweensaid washers by pump operation.

15. An axial piston type fluid pump comprising: a drive shaft; a casinghaving therein a plurality of cylinders arranged in axially opposedgroups comprising pairs of axially opposed cylinders incircumferentially spaced annular array, coaxial with and surroundingsaid shaft; pistons in said cylinders arranged in groups correspondingto the cylinder groups; a pair of wobble cams surrounding and secured tosaid shaft and disposed at opposite ends of said cylinders, said camshaving annular pistonoperating faces disposed in planes that areinclined in opposite directions with reference to the axis of saidshaft, with their respective high points disposed in axially opposedalignment; the respective pistons having at their outer ends meanscooperating with said cam faces for eifecting reciprocation of thepistons; said casing having an inlet and a delivery outlet for hydraulicfluid; valve means interposed between the respective groups of cylindersand embodying an inlet valve disposed for controlling fiow from saidinlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet; a pair ofbearing washers of synthetic resin plastic sheet material having aresiliency and wear resistance equivalent to that of the phenolicaldehyde resins, said washers having radial bearing faces provided withface grooves extending radially from their inner to their outer marginsand separating said bearing faces into radial bearing lands, and saidbearing washers having their opposite radial faces mounted against therespective cams; metal thrust washers fixed in the respective ends ofsaid casing and having radial bearing faces in bearing engagement withsaid bearing lands; and means providing a path for flow of hydraulicfluid to the inner extremities of said grooves, said lands, as theyrotate against said thrust washers, being slightly compressed by thelubricant entering between their leading edges and the thrust washerbearing faces, whereby there is formed a plurality of wedge-shapedspaces of minute depth, loaded with hydraulic fluid which supports theend thrust loads developed between said washers by pump operation.

16. An axial piston type fluid pump comprising: a drive shaft; a casinghaving therein a plurality of cylinders arranged in axially opposedgroups comprising pairs of axially opposed cylinders incircumferentially spaced annular array, coaxial with and surroundingsaid shaft; pistons in said cylinders arranged in groups correspondingto the cylinder groups; a pair of wobble cams surrounding and secured tosaid shaft and disposed at opposite ends of said cylinders, said camshaving annular piston-operating faces disposed in planes that areinclined in opposite directions with reference to the axis of saidshaft, with their respective high points disposed in axially opposedalignment; the respective pistons having at their outer ends meanscooperating with said cam faces for effecting reciprocation of thepistons; said casing having an inlet and a delivery outlet for hydraulicfluid, valve means interposed between the respective groups of cylindersand embodying an inlet valve disposed for controlling flow from saidinlet to respective cylinders and an outlet valve disposed forcontrolling flow from respective cylinders to said outlet; said pistonsbeing fitted to the cylinders so as to have minimum clearance therein atthe maximum temperature of the range of temperatures within which thepump is to operate, and so as to have maximum clearance in the cylindersat the minimum temperature of said range, said cylinders being of amaterial having a lower coefficient of expansion than the material ofsaid piston, whereby, with an increase in temperature, the pistons willexpand at a faster rate than the cylinders to compensate for thedecrease in viscosity of the working fluid resulting from suchtemperature increase, said coeflicients of expansion being soproportioned that the change in clearance will bear approximately arelation to the change in viscosity such that the fluid sealingefficiency of the piston in the cylinder, as translated into terms ofvolumetric efiiciency of the pump in high pressure operation, will bemaintained approximately a constant factor.

17. Control mechanism for a two-stage hydraulic pumping system,utilizing a constant output, low pressure supercharger, a variableoutput, automatically feathering high pressure pump, a connecting fluidline for delivery of fluid to the inlet of said high pressure pump, andmeans defining fluid passages for delivering the output of said highpressure pump to a load and for returning spent fluid from the load tothe inlet of said supercharger, said control mechanism comprising: acombined volume control and bypass valve unit including a casing havinga bore, an end member closing one end of said bore and an axiallyadjustable abutment member closing the other end of said bore, saidcasing further having an inlet and a delivery outlet both spaced fromsaid end member, said delivery outlet communicating with said connectingline, and a bypass outlet spaced from said inlet in the direction awayfrom said end member; a volume control valve comprising a controlpiston, a bypass piston and a stem connecting said pistons in axiallyspaced relation, said valve being slidable in said bore with saidcontrol piston disposed between said end member and said inlet anddelivery outlet and with said bypass piston disposed between said bypassoutlet and said adjustable abutment member; a spring interposed betweensaid bypass piston and said adjustable abutment member and loading saidvalve; one end of said control piston cooperating with .said end memberto define in said bore a control chamber, in which fluid pressure mayact against said control piston to balance the load of said spring andto shift the valve in response to fluid pressure changes in said controlchamber, the other end of said control piston being disposed forthrottling said delivery outlet in response to pressure increase in saidcontrol chamber; an inlet receiving fluid from the outlet of saidsupercharger, said volume control valve functioning for metering theflow from said inlet to aid connecting line and bypassing the excess tosaid bypass outlet; said casing further having a bypass leading fromsaid inlet to said bypass outlet, a pressure responsive bypass valve insaid casing, normally closing said bypass and adapted to open so as tounload the supercharger by bypassing back toward the supercharger inlet,that portion of the supercharger output which is in excess of the amountaccepted by said volume control valve; and a pilot valve responsive tofluctuations in the output pressure of said high pressure pump, forsupplying to said volume control valve, pressure fluid derived from saidhigh pressure pump output, for actuating said volume control valve in amanner to throttle the flow from References Cited in the file of thispatent UNITED STATES PATENTS Covert Aug. 11, 1931 Wahlmark Apr. 28, 1942'20 Deschamps Sept. 15, 1942 Somes Feb. 1, 1944 Parilla Oct. 14, 1947Deschamps Dec. 2, 1947 Born Aug. 26, 1952 FOREIGN PATENTS Great BritainFeb. 25, 1935

